Band selection

This prompts the user to select which band from the downloaded data to use. Gives them a list of available bands, user then selects which one they want.

I think this is a better way to go about band selection than scraping those pages and hoping that what we scraped can somehow be finagled into what bands are actually available in the data.

A further improvement on this would include a "default" flag at the command line that will just take the first band since that is usually the one that people want anyways. Not familiar with argparse at the moment, but could work on that if you all think this is a better way to go about this.

Example commands to show that this works outside of the MCD12Q1 data

`python loader.py download --dataset MCD45A1.005 --date 2007.04.01 --tiles tiles4 --zoom 3-10 --bbox 24 -88 31 -77`

Warning - that map is not interesting at all, but will download fairly quickly.